Prior to the present invention, equipment for regulating electrically regulated brake circuits of a multi-circuit brake mechanism and method of operating such brake circuits have been taught in the prior art. One such method and braking system is taught, for example, in FIG. 2 of EP-B-0 088 911. In this prior art publication, each of the brake circuits is capable of being regulated electrically as well as by fluid pressure signals. In this prior art system, these pressure signals are at relay valves 30, 31, 37, and 38. Because a brake pressure modulator used in this system is a rather expensive device, as a result of its complexity, the disclosed brake circuits are also expensive. This is particularly the case because at least two such brake pressure modulators are required for this system.
In a multi-circuit brake mechanism consisting of at least one electrically regulated brake circuit and at least one brake circuit that is regulated by the pressure signal, as taught in the prior art, the disadvantage exists that the pressure buildup of the electrically regulated brake circuit generally will lead the pressure buildup of the pressure regulated brake circuit. This condition normally will result in a non-uniform wear of the brake linings of a vehicle or, in the case of a vehicle combination, of another vehicle. Such vehicle combination, for example, being a tractor-trailer combination.